Recently, various enterprises and universities are actively engaged in the development of electronic paper. The most promising application of electronic paper is electronic books, and other applications include sub-displays of mobile terminals and display sections of IC cards. One of the most advantageous display method used for electronic paper is the use of a liquid crystal display panel utilizing a cholesteric liquid crystal. A liquid crystal display panel utilizing a cholesteric liquid crystal has excellent features such as semi-permanent display retention characteristics (memory characteristics), vivid color display characteristics, high contrast characteristics, and high resolution characteristics. A cholesteric liquid crystal is obtained by adding a relatively great amount of chiral additive (chiral material) to a nematic liquid crystal (to a chiral content of several tens percent), and it is also called a chiral nematic liquid crystal. A cholesteric liquid crystal forms a cholesteric phase in which nematic liquid crystal molecules are greatly twisted helically to such a degree that incident light will undergo interference reflection.
A liquid crystal display panel utilizing a cholesteric liquid crystal displays an image by controlling the alignment of liquid crystal molecules at each pixel. States of alignment of a cholesteric liquid crystal include a planar state and a focal conic state. Those states exist with stability even when there is no electric field. A liquid crystal layer in the focal conic state transmits light, and a liquid crystal layer in the planar state selectively reflects light rays having particular wavelengths in accordance with the helical pitch of the liquid crystal molecules. A liquid crystal display panel utilizing a cholesteric liquid crystal cannot be properly made to enter the planar state or focal conic state unless a pulse voltage having an optimal pulse width is applied.
A liquid crystal display panel of this type, which may be used as a display panel of a liquid crystal display, is vulnerable to the influence of temperatures. For example, the viscosity of a cholesteric liquid crystal increases in a manner like an exponential function at low temperatures, which results in a corresponding reduction in the response (γ characteristic) of the cholesteric liquid crystal to a pulse voltage applied to drive the same. For this reason, it is necessary to vary the magnitude or pulse width of the pulse voltage applied to drive the cholesteric liquid crystal depending on temperatures. FIG. 9 illustrates examples of optimal effective pulses widths (ms) associated with temperatures a common cholesteric liquid crystal. Temperatures (C.°) of the cholesteric liquid crystal is illustrated along the horizontal axis, and logarithms of the optimal effective pulse widths (ms) are illustrated along the vertical axis. Referring to FIG. 9, points D1 to D7 represent actual effective pulse widths at respective temperatures, and a curve C is a curve approximated from the points D1 to D7. As depicted in FIG. 9, the optimal effective pulse width applied to the cholesteric liquid crystal is greater, the lower the temperature of the cholesteric liquid crystal. The figure also depicts that the pulse width is smaller, the higher the temperature.
FIG. 10 is an illustration schematically depicting display irregularities of a liquid crystal display. As depicted in FIG. 10, a liquid crystal display panel 246 of a liquid crystal display may enter a state in which pixel regions at relatively high temperatures and pixel regions at relatively low temperatures coexist (a state what is called temperature irregularities) because of heat emitted from a driving circuit 231, a control circuit 232, and the like provided on a circuit substrate 230 of the panel or heat from not-depicted devices provided inside the liquid crystal display. In such a state, the liquid crystal display panel 246 has the problem of so-called display irregularities attributable to differences in response to substantially the same pulse voltage between the pixel regions at high temperatures and the regions at low temperatures. In order to reduce display irregularities attributable to temperature irregularities as thus described, liquid crystal displays utilizing a cholesteric liquid crystal have been proposed, in which a heat leveling layer is provided on a bottom surface of a light absorbing layer to allow uniform heat radiation substantially throughout the bottom of a display surface (for example, see JP-A-2002-82325).
In such a liquid crystal display, although display irregularities attributable to temperature irregularities are reduced, the temperature of the liquid crystal display panel itself is not increased. Therefore, no improvement is achieved in the response of the liquid crystal. Therefore, a problem arises in that degradation of the response of the liquid crystal results in an extremely long screen rewriting time especially at low temperatures. FIG. 11 is an illustration schematically depicting a sectional configuration of a liquid crystal display having an electric heater. In order to solve the above-described problem, for example, a configuration as depicted in FIG. 11 may be employed. Specifically, a film-like electric heater 212 generating heat from electric power supplied by a battery 213 is provided on a bottom surface of a liquid crystal display panel 246, whereby the liquid crystal layer is heated to improve the response of the liquid crystal.
The above-described configuration including an electric heater provided on a bottom surface of a liquid crystal display panel has a problem in that high electric power is consumed to generate heat by the electric heater. In particular, when a liquid crystal display having such a configuration is provided in a battery-driven electronic apparatus, a problem arises in that power dissipation of the battery is significantly accelerated.